The present invention generally relates to fatty acid esters, and more particularly to a method for lowering cloud point of the fatty acid esters. A multitude of energy crises have been caused by disruption of fossil fuel supplies, coupled with significantly increased demand for fossil fuels by industrialized nations. In the past few decades, these crises have encouraged the development of alternative fuels. Additionally, since there are finite reserves of crude oil from which petroleum-based fuels are derived, there has also been a trend toward developing renewable fuels, such as biodiesel, which is derived from renewable sources.
Soy methyl ester (SME) or methyl soyate, the chemical description of which is provided below, is a common organic acid ester precursor for producing biodiesel. Organic acids, as the name indicates, are organic compounds with acid-like properties. One common group of organic acids are carboxylic acids, which have a “—COOH tail.” Esters constitute a class of organic acid compounds where at least one —OH member is replaced by an alkoxy group (—O—CnH2n+1). In the case of methyl acetate ester, for example, a methoxy group (—O—CH3), which is the simplest form of an alkoxy, has replaced the —OH group in acetic acid CH3COOH. This results in CH3COOCH3, or methyl acetate ester. This chemical reaction is commonly termed esterification. Diagram 1, found below, shows the chemical bond structures for these compounds.

Fatty acids consist mainly of carbon chains and hydrogen atoms. These chains can be short with a small number of carbon atoms, e.g., butyric acid (CH3CH2CH2COOH), or long with large number of carbon atoms, e.g., oleic acid CH3(CH2)7CH═CH(CH2)7COOH. Fatty acids may include single and double bonds between carbon atoms. A saturated fatty acid has the maximum number of hydrogen atoms covalently bound to each carbon atom in the chain of carbon atoms, i.e., a saturated fatty acid has no double bonds. An unsaturated fatty acid has at least one double bond between two carbon atoms. Diagram 2, found below, shows an example of saturated and unsaturated fatty acids.

SME is produced by the transesterification of soybean oil with methanol in the presence of a catalyst. Transesterification results in a class of organic reactions where one ester is transformed into another ester by interchanging at least one alkoxy. The catalyst is often an acid or base. For example, methanol is added to NaOH and added to soybean oil to separate fatty acid esters from glycerin. In this example, the mixture of NaOH, methanol, and soybean oil is often heated to accelerate the esterification step.
SME profile by percent and by molecular weight is given in Table 1, below.
TABLE 1Typical SME profileFattyMolecularMeltingPercent ofAcidCarbonWeightPointSME byNameDesignFormula and Structure(g/mole)(° C.)Weightmethyl palmitateC16:0C15H31CO2CH3   270.530.510.3 methyl stearateC18:0C17H35CO2CH3   298.539.14.7 methyl oleateC18:1C17H33CO2CH3   296.5−19.822.5 methyl linoleateC18:2CH3(CH2)4CH═CHCH2CH═CH(CH2)7CO2CH3   294.5−34.954.1 methyl linolenateC18:3CH3(CH2CH═CH)3(CH2)7CO2CH3292.5−57.08.3
Biodiesel produced by typical methods suffers from a crystallization phenomenon when temperatures decrease. Although this crystallization phenomenon is not limited to biodiesel, the temperature at which biodiesel begins to crystallize is substantially higher than petroleum-based diesel fuel. The crystallized constituents can clog fuel filters in vehicles using biodiesel and thereby cut off the fuel supply to the engine. The temperature at which solids begin to precipitate, thus producing a cloudy mixture, is referred to as cloud point (C.P.). Saturated fatty acid ester constituents crystallize at a higher temperature than unsaturated fatty acid esters. To lower the temperature at which crystallization occurs and thereby lower the C.P. of the fuel, several techniques can be used, examples of which include blending with petroleum-based diesel, introducing additives, and winterization. Winterization refers to crystallization and removal of saturated fatty acids, e.g., C16:0 and C18:0, and in some cases mono-unsaturated fatty acids, e.g., C18:1, that cause the biodiesel product to crystallize at an undesirably high temperature. The crystallization process is typically performed by cooling, and the removal process is typically performed by filtration of the crystallized particles of the saturated and in some cases mono-unsaturated fatty acids which leaves a mixture having a greater amount of polyunsaturated fatty acids compounds with lower C.P., thereby lowering the C.P. of the biodiesel so produced.
The winterization process has gained more interest in recent years. Winterization by itself produces low yields, i.e., a substantial portion of the starting material is lost during the filtration process. Therefore, use of compounds which improve the winterization process, typically referred to as “improvers,” is essential. One process which includes the addition of improvers is referred to as fractionation, which uses the crystallization properties of esterified fatty acids to separate a mixture into low and high C.P. liquid fractions. During fractionation, these improvers create inclusion compounds/complexes. The process of creating an inclusion compound involves a host constituent, namely, the fatty acid molecule, which has a series of cavities or landing sites for attachment by a second chemical constituent, commonly referred to as the guest, which is an improver compound. The compound resulting from the combination of the host and the guest is called an “inclusion compound.” The forces that hold the host and the guest constituents together are van der Waals type forces. That is, covalent bonds do not typically form between the guest and the host. Clathrates are one type of inclusion compound in which the spaces in the host constituent are enclosed on all sides, causing a “trapping effect.”
A common improver compound is urea. Urea selectively forms clathrates with fatty acid molecules. Initially, urea forms clathrates with longer straight chain saturated fatty acid molecules, e.g., C18:0. As the number of longer chain saturated fatty molecules decline, urea then forms inclusion compounds with shorter straight chain saturated fatty acids molecules, e.g., C16:0, and then with mono-unsaturated fatty acid molecules that are nonlinear, e.g., C18:1. The reason for this selectivity is thought to be the ease of clathration observed with longer linear chain saturated fatty acids due perhaps to these moleuces possessing a larger number of landing sites for the urea molecule.
There are three different types of fractionation: dry fractionation, detergent fractionation, and solvent fractionation. Solvent fractionation has received a substantial amount of interest in recent years. The key to efficient fractionation is to thoroughly mix the improvers, e.g., urea, with the fatty acids. Solvent is used as a carrier for the improver. That is, the improver is dissolved in the solvent and the solvent-improver combination is added to the fatty acid to make a homogenous mixture.
Addition of solvent-improver to the fatty acid esters alone, however, does not promote formation of clathrates. For example, if urea is dissolved in methanol and the urea-methanol mixture is added to a mixture of fatty acid esters to form a homogenous mixture, urea molecules preferentially stay dissolved in methanol rather than forming clathrates with the fatty acid esters. In order to initiate the desired clathration, a change in conditions must occur. In the prior art, in order to begin the clathration, the homogenous mixture is cooled. As the homogenous mixture is cooled, urea molecules begin to form clathrates based on the selectivity described above. The clathrates crystallize and can be separated by filtration, centrifugation, etc. As the homogenous mixture is formed the temperature is often elevated. This is due to heating in the transesterification phase in order to accelerate the esterification step, and since the homogenous mixture is often formed directly after the transesterification phase. The desired C.P. is directly proportional to how much the homogenous mixture is cooled. For ultralow C.P.s, the target cooling temperature is very low. Thus, due to a thermodynamic equilibrium, clathration formation slows unless the temperature is further reduced. In order to form additional clathrates, the temperature must be further reduced.
However, clathration by cooling has several disadvantages. First, cooling the homogenous mixture is costly and in some cases not possible. For example, in many developing countries, the requirement to cool the homogenous mixture is prohibitively expensive due, for example, to high ambient temperature under some conditions. Second, cooling takes a substantial amount of time. Large batches of fatty acid esters have correspondingly large thermal masses. Therefore, an undesirably long time may be required to reach the target cooling temperature. Even if the homogenous mixture is allowed to naturally cool to room temperature, and thereby eliminate the need for active cooling, a substantial amount of time may be required due to the thermal mass. Also, relying on naturally cooling to ambient temperatures may result in inconsistent output quality because the target cooling temperature may vary and the target temperature plays a key major role in the final product's C.P. Third, in cooling based processes involving typical target temperatures, some urea almost invariably stays dissolved in the solvent instead of forming clathrates. To quantify how much urea is used, urea utilization is defined as a ratio of urea that forms clathrates to the total amount urea supplied in the process. In the cooling process, urea utilization is always inherently below 100%. Fourth, the cooling process requires eventual separation of methanol from the unsaturated-rich fatty acid esters. This is required for commercial biodiesel because of industry standards set for the final product which limit the amount of methanol in biodiesel, e.g., to 0.2% by volume.
What is needed is a robust, fast and efficient method of lowering the cloud point of Biodiesel that overcomes the drawbacks of the art discussed above.